This application is based upon and claims the benefit of priority of Japanese Patent Application No. H.11-268964 filed on Sep. 22, 1999, the content of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a sparkplug for internal combustion engine having a better self-cleaning function in use of surface creeping spark discharges.
2. Description of Related Art
Recently, as environmental preservation has become more important, stratified charge internal combustion engines with lower fuel consumption have been widely used as more environmentally-friendly engines.
However, when stratified fuel mixtures are burned in a combustion chamber, rich fuel mixtures are concentrated near a spark plug so that the spark plug may tend to be smoldered or fouled by carbon. The carbon-fouling makes the insulation property of an insulator surrounding a center electrode worse so that a spark discharge may not occur across the regular discharge gap provided between center and ground electrodes but instead may occur between the insulator, on a surface of which carbon is deposited, and the inside of a metal housing for mounting at a portion deep into the metal housing from a front end surface of the insulator.
To cope with this problem, there are known self-cleaning spark plugs as disclosed in JP-Y2-53-41629 or JP-A-47-19236.
According to JP-Y2-53-41629, the spark plug has a plurality of electrodes constituting first and second ground electrodes. A first discharge gap is formed between the first ground electrode and the center electrode and a second discharge gap is formed between the second ground electrode and the center electrode. A regular spark discharge occurs through the first discharge gap and, when the insulator is fouled by carbon deposit, a spark discharge occurs through the second discharge gap, not through the portion deep into the metal housing, so that carbon may be burned without decreasing the ignition capability of the spark plug.
Further, according to JP-A-47-19236, there are provided the regular first discharge gap and the second discharge gap through which sparks are discharged when the insulator is fouled. It is characterized, in this case, that a front end of the center electrode is nearly equal in height to a front end of the insulator.
Therefore, as the spark discharge at the first discharge gap occurs at a position nearly same in height as the second discharge gap, it is contemplated that the respective ignition characteristics at both first and second discharge gaps do not have much difference.
However, the spark plug according to JP-Y2-53-41629 has a drawback in that there exists a big difference of ignitability between the respective spark discharges at the first and second discharge gaps, since the second discharge gap formed at a leading end of the metal housing is arranged at a position far away from the first discharge gap, so that drivability is adversely affected, in particular, in stratified fuel combustion.
Further, as the spark discharge at the second discharge gap occurs at a place deep from the leading end of the insulator into an insulator base, channeling is likely to occur.
On the other hand, according to the spark plug disclosed in JP-A-47-19236, there is also a problem that ignitability is not good, as the front end of the first electrode is obliged to be almost same in height as the front end of the insulator so that the front end of the insulator may operate to cool flame cores generated by spark discharge at the first discharge gap. Further, though carbon adhered to the surface of the insulator is burned by creeping discharge along the surface of the insulator through the second discharge gap, there is a problem that a channeling is likely to occur so that grooves may be formed on the surface of the insulator as shown in FIGS. 12A and 12B. FIG. 12A shows the insulator of the spark plug viewed in an axial direction of the center electrode and FIG. 12B shows the insulator in side view.
The present invention has been made in view of the above mentioned problem, and an object of the present invention is to provide a spark plug for internal combustion engines in which a remarkably longer life time of fouling resistance is secured in such a manner that an air-gap spark discharge with a good ignitabilty usually occurs at a first discharge gap and, when the insulator is fouled, a surface creeping spark discharge occurs at a second discharge gap to burn carbon deposited on the surface of the insulator, while the channeling on the surface of the insulator is limited.
To achieve the above object, the spark plug has a center electrode, first and second ground electrodes, an insulator and a metal housing. The first discharge gap is constituted between a front end of the center electrode and a front side of the first ground electrode and the second discharge gap is constituted between a front end of the second electrode and a front side of the center electrode.
In this case, dimensional relationships of the center electrode, the first and second ground electrodes, the insulator and the metal housing are respectively in ranges of,
Axe2x89xa6(C+D)+0.5B
Bxe2x89xa70.6 mm
Cxe2x89xa61.6 mm
Hxe2x89xa70.9 mm
where
A is a distance of the first discharge gap,
B is a radial thickness of the front end of the insulator
C is a shortest distance between the insulator and the front end of the second electrode,
D is a shortest distance between a base point of the center electrode where a diametrically reduced portion thereof is integrally connected to a base portion thereof and the inside front end of the insulator, and
H is a distance of spark discharge creeping along the end surface of the insulator.
Further, it is preferable that dimensional relationships of the center electrode, the first and second ground electrodes, the insulator and the metal housing are respectively in ranges of,
Axe2x89xa6(C+D)+0.5B
Bxe2x89xa70.6 mm
Cxe2x89xa61.6 mm
Exe2x89xa60
xe2x88x920.5 Gxe2x89xa6Fxe2x89xa60
|B|+|E|+|F|xe2x89xa71.2 mm
where
A is a distance of the first discharge gap,
B is a radial thickness of the front end of the insulator,
C is a shortest distance between the insulator and the front end of the second electrode,
D is a shortest distance between the base point of the center electrode and the inside front end of the insulator,
E is an axial length from the inside front end of the insulator to the base point of the center electrode (shown as + mark if the base point protrudes out of the inside front end of the insulator),
F is an axial length from the front end of the insulator to the front end of the second electrode on a side of the housing (shown as + mark if the front end of the second electrode on a side of the housing protrudes out of the front end of the insulator, and
G is an axial thickness of the front end of the second electrode.
To realize a spark plug having a longer consumption life time, it is preferable that at least one of the front end of the first ground electrode and the front end of the center electrode is provided with a noble metal chip preferably made of any one material of pure Pt, pure Ir, Pt alloy and Ir alloy.